Acid-resistant enamel for stainless steel



,0 4,816 AClD-RESiSTANT ENAgiLEL FOR STADILESS Eurnham W. King and Marion C. Broekway, Columbus,

Ohio, msignors, by mesne assignments, to Union Carbide Corporation, Niagara. Falls, N.Y., a corporation of New York No Drawing. Filed Get. 21, 1959, Ser. No. 847,692

9 Claims. (Cl. 117-129) This invention relates to enamel coating materials and, more particularly, to an enamel coating material for use on stainless steels.

Stainless steel structures and panels are often coated with ceramic-type coatings to afford corrosion resistance and decorative effects. However, the lack of suitable coating materials has hindered the development of this new architectural material. For example, the ceramic coating must be applied in a layer having a thickness on the order of 4 mils or more if a true, deep coloring is to be obtained. ings afford greater protection to the underlying steel structures from weather attack. However, despite the need for thicker coatings, present day coatings are applied in thin layers, often less than 2 mils, because of the lack of adherence of thicker coatings to the steel subsurface. '1' his failure of the presently produced thicker coatings to adhere firmly to the steel results in cracking and spalling of the coating, especially when the underlying steel is subject to stress.

It is the primary object of this invention, therefore, to provide an enamel coating composition which can be applied to stainless steel as a coating of a greater thickness than heretofore produced.

It is also an object of this invention to provide an enamel coating composition for stainless steel which afiords greater protection to the underlying steel because of its excellent acid and weather resistance.

It is a further object of this invention to provide an enamel coating composition for stainless steels which yields a ceramic coating of a predictable color and which is amenable to changes in color through the addition of pigments.

It is another object of this invention to provide an article of manufacture comprising a stainless steel article having a ceramic coating.

Other aims and advantages of this invention will be apparent from the following description and appended claims.

In accordance with the present invention, an enamel coating composition is provided consisting essentially of from about 45 to about 85 parts by weight silica, from about 2 to about 25 parts by weight vanadium pentoxide, from about 17 to about 27 parts by weight of at least one of the oxides of sodium and potassium, up to 25 parts by weight titanium dioxide, up to 12 parts by Weight calcium oxide, up to 12 parts by weight boron oxide, and up to 20 parts alumina, the boron oxide and alumina being present in a minimum combined amount of at least 5.5 parts by weight.

In a preferred composition of the coating material the constituents are present in the percentage ranges given in Table I. It will be noticed that a minimum total amount of B and/or A1 0 of 5.5 percent by weight is required.

Constituent: Table I Parts by weight SiO 50 to 75 V205 7 to Na O and/or K 0 19 to 23 TiO 2 to 15 CaO 0 to B 0 0 to 6 Q1203 0 to 7 Furthermore, thicker ceramic coat- The oxide enamel compositions given above contain as the principle glass-forming oxides Si0 and E 0 Included in the mixture should be one or more alkalies, such as Na O and K 0 in at least the minimum amounts specified. The chemical stability of the enamel is improved by the addition of A1 0 TiO and CaO. Additionally it is required that there be a minimum combined amount of 3 0 and Al O of 5.5 parts by weight to avoid water solubility. The TiO addition is effective in improving the acid resistance.

The addition of V 0 to the enamel composition serves to increase the degree of enamel adherence to the steel subsurface thus making possible the application of thicker coatings. The V 0 addition also produces a greater degree of acid resistance in the enamel.

The enamel coating compositions described above are produced from a batch composition which is smelted to form a fluid glass. The glass is then quenched to obtain an enameling frit, or coating composition. This frit is then applied to the surface of an article, such as a stainless steel article, which is tired in such manner as to transform the frit into an enamel coating.

The raw materials should be smelted within the temperature range of 2500" to 2800 F. for from 20 to 60 minutes. The resulting glass composition is quenched to form a frit and the frit is ground with clay and water to produce a slip having the desired consistency. The slip is then applied to a stainless steel surface in any suitable manner, such as by painting, dipping or spraying. The firing should preferably be performed at temperatures within the range of 1600 to 1700 F. for from 5 to 10 minutes. The temperature range for enameling, however, may be broadened to a temperature range of from l300 to 1850 for from 3 to 20 minutes. Enamel coatings on stainless steel surfaces may thus be produced which have substantial thickness, as for instance on the order of 5 to 10 mils, and which have good chemical durability, and which adhere firmly to the stainless steel.

An enamel coating produced within the above preferred compositional range is characterized by Class AA acid resistance. Class AA acid resistance is the designation of the Porcelain Enamel Institute for materials which pass the following test: specimens are exposed to a 10 percent aqueous solution of citric acid at room temperature for a period of 15 minutes. If no stain is visible and if the specimen passes the dry rub test, the enamel is considered to be Class AA acid resistant. The dry rub test is the following: the specimen is subjected to pencil marking with a No. 1 lead pencil across both the acidtreated areas of the specimen and the non-treated areas. The specimen is then rubbed with a clean dry cloth. If the marks are retained more tenaciously on the acidtreated area than on the non-treated area, the specimen fails the test. Y

Enamel coatings on stainless steel produced within the preferred range are further characterized by good enamel fit and fair or better adherence. Good enamel fit refers to the enamel having a coefficient of thermal expansion equal to or a little less than that of stainless steel. With changes in temperature, the enamel and metal expand and contract together and there is no danger of spontaneous chipping or cracking of the enamel.

Adherence refers to the tenacity with which the enamel adheres to the steel subsurface, especially when the steel i stressed, and is rated from excellent to good, better, fair, poor, and no adherence. The adherence characteristics can be determined visually by observing the amount of coating still adhering to an enameled sheet after the sheet is bent or hit with a hammer. When the steel is stressed sufiiciently to cause the metal to deform plastical- 1y, as by a hammer blow, the enamel will crack. If the Table V Theoretical composition,

Example No.

parts by weight 7.7 2.0 2.0 3.5-.-- 10.0.-- Enamclmg conditions:

Temperature,F. 1,700 1,650 1,700.." 1,700. Tune at temperature, 1 l0 10.

mm. Enamel properties:

Adherence Fair Fain F ood air Excellent Poor.... Poor. Color Dark khaki. Khaki. Brown-black- Dark khaki. Brown-black Green..- Khaki-gray. Acidresistance AA AA... AA AA AA. AA.

rom the data presented above, it is readily apparent that Class AA acid resistance was achieved in each instance and that enamel compositions which fell within the preferred range (Examples 1 to 7) possessed fair to excellent adherence of the enamel coating to the stainless steel surfaces. Examples 8 and 9, while showing only poor adherence, still possess a high degree of usefulness.

In still another series of experiments, the effect of V 0 additions with respect to adherence promotion on stainless steel was tested. Frit compositions having the oxide mixtures shown in Table VI were prepared. These frits were applied to Types 202 and 302 stainless steels at the firing temperatures and times shown in Table VII. The degree of adherence is also shown in Table VII.

black, and probably red enamels could be produced by utilizing the compositions falling within the scope of the present invention as the base.

The present invention, which comprises the preparation of V O -containing frits for coating stainless steel, thus represents an important advance in the enamel industry. Thicker coatings of enamel, on the order of 5 to 10 mils, are capable of application on stainless steel surfaces by virtue of the superior adherence characterizing the enamels of the present invention. In addition, the excellent acid and weather resistance of the present enamel coatings on stainless steel makes possible a greater flexibility of use for enameled stainless steel panels, as for instance, for architectural purposes.

What is claimed is:

1. An enamel frit consisting essentially of from about to about 85 parts by Weight SiO from about 7 to about 25 parts by weight V 0 from about 17 to about 27 parts by weight in the aggregate of at least one alkali oxide selected from the group consisting of Na O and K 0, from about 2 to 25 parts by weight TiO up to 12 parts by weight CaO, up to 12 parts by weight B 0 and up to 20 parts by weight A1 0 the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight.

Adherence characteristics of hits containing varying amounts of V 05 Example No. Type 302 stainless steel, temperature, F. (10 minutes) Type 202 stainless steel, firing temperature, F.

Negligible.-- Poor-fair. Poor- Fair. Fair-good Good Poor. Po01 P0or-fair. Very poor Good Excell Fair Fair Good. Poor .11 Good Good Do.

Frits were also prepared having compositions similar to those of Examples 10 and 13 of Table Vi but which contained 30 or 40 percent by weight V 0 in each instance, less than Class AA acid resistance resulted. It

was also noted that the enamel coating containing 40 percent by weight V 0 became white and opaque during firing.

Although chemically pure vanadium pentoxide was used in the above test, other V 0 sources, such as red cake, may be employed with equal effectiveness with respect to adherence promotion and acid resistance pro viding the batch composition is suitably corrected and enough red cake is added so that the vanadium oxide composition of the frit is in accordance with the ranges 79 of ceramic pigments as mill additions, blue, green, brown,

2. An enamel frit consisting of from about 50 to about 75 parts by weight SiO from about 7 to about 20 parts by weight V 0 from about 19 to about 23 parts by weight in the aggregate of at least one alkali oxide selected from the group consisting of Na o and K 0, from about 2 to about 15 parts by weight TiO up to about 10 parts by weight CaO, up to about 6 parts by weight B 0 and up to about 7 parts by Weight A1 0 the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight.

3. An enamel frit consisting essentially of about 15 parts by weight V 0 about 18.4 parts by weight Na O, about 2.6 parts by weight K 0, about 8.5 parts by weight TiO about 5 parts by weight CaO, about 3 parts by weight B 0 about 3.5 parts by Weight A1 0 and the balance substantially all Si0 in an amount of about 62.5 parts by weight and incidental impurities.

4. A method of producing an adherent, acid-resistant coating on a stainless steel surface comprising pulverizing an enameling frit consisting essentially of from about 45 to about 85 parts by weight SiO from about 7 to about 25 parts by weight V from about 17 to about 27 parts by Weight in the aggregate of at least one alkali oxide selected from the group consisting of Na O and K 0, from about 2 to 25 parts by weight TiO up to 12 parts by weight (3216, up to 12 parts by weight B 0 and up to 20 parts by weight A1 0 the minimum amount in the aggregate of B 0 and'Al O being at least 5.5 parts by weight, applying said frit on the surface to be coated, and firing it at a temperature between about 1300 F. and about 1850 R, for from about 3 to about 29 minutes to obtain an enamel coating.

5. A method of producing an adherent, acid-resistant coating on a stainless steel surface comprising pulverizing an enameling trit consisting essentially of from about 45 to about 85 parts by weight SiO from about 7 to about 25 parts by weight V 0 from about 17 to 27 parts by weight in the aggregate of at least one allrali oxide selected from the group consisting of Na O and K 0, from about 2 to 25 parts by Weight TiO up to 12 parts by Weight CaO, up to 12 parts by Weight B 0 and up to 20 parts by weight A1 0 the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight, applying said frit on the surface to be coated, and firing it at a temperature between about 1600 F. and about l700 F. for from about 5 to about 10 minutes to obtain an enamel coating.

6. A method of producing an adherent, acid-resistant coating on a stainless steel surface comprising pulverizing an enameling frit consisting essentially of from about 50 to about 75 parts by weight SiO from about 7 to about 20 parts by weight V 0 from about 19 to about 23 parts by weight in the aggregate of at least one alkali oxide selected from the group consisting of Na O and K 0, from about 2 to about parts by weight TiO up to about 10 parts by weight CaO, up to about 6 parts by weight, B 0 and up to about 7 parts by weight A1 0 the minimum amount in the aggregate of. B 0 and A1 0 being at least 5.5 parts by weight, applying said frit on the surface to be coated, and firing it at a temperature between abou 1300 F. and about1850 F. for from about 3 to about 20 minutes to obtain an enamel coating.

7. A method of producing an adherent, acid-resistant coating on a stainless steel surface comprising pulverizing an enameling frit consisting essentially of from about 50 to about 75 parts by weight SiO from about 7 to about 20 parts by weight V 0 from about 19 to about 23 parts by weight in the aggregate of at least one alkali oxide selected from the group consisting of Na O and K 0, from about 2 to about 15 parts by weight T10 up to about 10 parts by weight CaO, up to about 6 parts by weight B 0 and up to about 7 parts by Weight A1 0 the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight, applying said frit on the surface to be coated, and firing it at a temperature between about 1600 F. and about 1700 F. for from about 5 to about 10 minutes to obtain an enamel coating.

8. An article of manufacture comprising a stainless steel member at least partially coated with an enamel coating formed by firing thereon an enamel frit consisting essentially of from about 45 to about 85 parts by weight SiO from about 7 to about 25 parts by weight 0 from about 17 to about 27 parts by Weight in the aggregate of at least one alkali oxide selected from the group consisting of Na i) and K 0, from about 2 to 25 parts by weight TiO up to 12 parts by weight CaO, up to 12 parts by weight 3 0 and. up to 20 parts by Weight A1 0 the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight.

9. An article of manufacture comprising a stainless steel member at least partially coated with an enamel coating formed by firing thereon an enamel frit consisting essentially of from about to about parts b weight SiO from about 7 to about 20 parts by weight V 0 from about 19 to about 23 parts by weight in the aggregate of at least one alkali oxide selected from the group consisting of N320 and K 0, from about 2 to about 15 parts by weight TiO up to about 16 parts by weight CaO, up to about 6 parts by weight B 0 and up to about 7 parts by Weight Al O the minimum amount in the aggregate of B 0 and A1 0 being at least 5.5 parts by weight.

References Cited in the file of this patent UNITED STATES PATENTS 2,575,470 Bennett et al. July 5, 1949 2,829,062 Bennett et al. Apr. 1, 1958 FOREIGN PATENTS 176,415 Austria Oct. 26, 1953 OTHER REFERENCES Tashi-mo et al.: J. Ceram. Assoc, Japan 61, 53740 (1953), Chem. Sbs. 48241949. 

4. A METHOD OF PRODUCING AN ADHERENT, ACID-RESISTANT COATING ON A STAINLESS STEEL SURFACE COMPRISING PULVERIZING AN ENAMELING FRIT CONSISTING ESSENTIALLY OF FROM ABOUT 45 TO ABOUT 85 PARTS BY WEIGHT SIO2, FROM ABOUT 7 TO ABOUT 25 PARTS BY WEIGHT V2O5, FROM ABOUT 17 TO ABOUT 27 PARTS BY WEIGHT IN THE AGGREGATE OF AT LEAST ONE ALKALI OXIDE SELECTED FROM THE GROUP CONSISTING OF NA2O AND K2O, FROM ABOUT 2 TO 25 PARTS BY WEIGHT TIO2, UP TO 12 PARTS BY WEIGHT CAO, UP TO 12 PARTS BY WEIGHT B2O3, AND UP TO 20 PARTS BY WEIGHT AL2O3, THE MINIMUM AMOUNT IN THE AGGREGATE OF B2O3 AND AL2O3 BEING AT LEAST 5.5 PARTS BY WEIGHT, APPLYING SAID FRIT ON THE SURFACE TO BE COATED, AND FIRING IT AT A TEMPERATURE BETWEEN ABOUT 1300*F. AND ABOUT 1850*F., FOR FROM ABOUT 3 TO ABOUT 20 MINUTES TO OBTAIN AN ENAMEL COATING. 